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The use of crop residues as surface mulches has been shown to have a beneficial effect on the growth of pearl millet [Pennisetum glaucum (L.) R.Br.] in the Sahelian zone of West Africa. This study was conducted to discern the mechanism(s) responsible for yield increases resulting from crop-residue additions in a field trial located on a sandy soil at the ICRISAT Sahelian Center near Niamey, Niger. Soil chemical and physical properties were examined to a depth of 120 cm after 5 yr of application of millet residue as a surface mulch, P and N fertilizer, or a combination of fertilizer and residue. Annual residue application resulted in a higher exchangeable-base content, lower Al saturation, and slightly higher available-P values than the control. The use of fertilizer resulted in an increase in available P and exchangeable Ca. The combination of residue plus fertilizer resulted in greater enhancement of soil fertility parameters than the use of these inputs alone. Differences in soil chemistry were operative in only the top 20 cm of the soil profile, however, the surfaces of the plots receiving residues were 15 to 20 cm higher than the surfaces of the control and fertilizer-only plots. The surfaces of the residue plots also had lower clay contents than the surface soils in the nonresidue plots. The increase in soil fertility following the application of millet residue as a surface mulch was due to two mechanisms: (i) the recycling of nutrient elements to the soil following termite and microbial decomposition of the residue, and (ii) the entrapment of eolian materials, which generally have better fertility characteristics than the subsoil, or protection of the more fertile surface soil from the erosive effects of the strong winds that are common in the Sahel.

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A study was carried out to determine soil morphological and chemical properties that influence short‐range variability in the growth of millet [Pennisetum glaucum (L.) R. Br.] in western Niger. Paired productive and adjacent nonproductive sites within millet fields were evaluated along transects in three soil‐geomorphic regions. Transects totaled >300 km in length. Mean depth to subsoil of productive sites ranged from 17.06 cm in the sand valley transect to 19.98 cm in the Dallol Bosso transect. These were different from the topsoil depths of the adjacent nonproductive sites within the respective transects, which ranged from 7.8 to 12.9 cm. Productive sites were located in microtopographic “high” positions relative to nonproductive sites. The mean relief differential between productive and nonproductive sites was 6.13 cm. Vesicular crusts 2 to 5 cm thick and exposed Bt horizon crusts (2.5YR 4/6) were mostly associated with nonproductive sites. The acidity parameters were highly intercorrelated in all transects. Mean pH of soils associated with productive growth of millet ranged from 5.8 to 6.3, compared with 5.4 to 6.0 in the nonproductive areas. Mean exchangeable acidity in the nonproductive sites ranged from 0.2 to 0.36 cmolc kg‐1, about three times higher than the mean in the productive areas. Bray‐1 P levels to a depth of 30 cm did not differ between paired sites in any of the transects. Microtopography was the most important factor influencing millet growth variability in western Niger because it directly influenced the factors related to crust formation and determined the depth to acidic, Al‐rich, P‐deficient subsoil that impeded millet growth and development.

Variability in the growth of Faidherbia albida (Del.) A. Chev. (syn. Acacia albida [Del.]) in the Sahelian zone of West Africa is a common observation. The "albida effect" has been attributed to the amelioration of soil chemical, physical, and microbiological properties by the direct action of the tree, resulting in enhanced crop production beneath the mature canopy. Crop yields within fields without F. albida can also be highly variable within short distances. A study was conducted to observe the association between the variability in soil properties and the variable growth of young (2-yr-old) F. albida trees planted at the ICRISAT Sahelian Center located near Niamey, Niger. The results of this study indicate that the variability in F. albida growth is caused, in large part, by variability in soil properties across relatively short distances. Good growth sites had higher clay contents and base saturations, and lower exchangeable acidity than poor growth sites within the field. The tallest trees in the field were associated with the proximity to abandoned termite mounds and other microhigh sites. Seedlings planted on sheet-eroded sites caused by runoff from microtopographical high sites within the field grew poorly. The variability in F. albida growth in plantations within Niger is hypothesized to be due, in part, to preexisting soil conditions. These results also suggest that the albida effect might be partially caused by these preexisting "islands of fertility". This site-determined variable growth ofF. albida could be exploited with proper seedling placement strategies.